The present invention relates to a gauge plate for determining and measuring the actual locations of the guide pins for the lower internal structure in a nuclear reactor vessel for use in customizing a replacement upper internals structural package, and to a method of using this gauge plate. More particularly, the present invention relates to a gauge plate, and a method of using same, whereby the actual position and dimensions of the existing guide pins in the barrel of a nuclear reactor vessel for the lower internal structure can be determined so that the conventional inserts for the upper core plate which mate with these guide pins can be customized to the required close tolerances when the upper internals structural package for an existing nuclear reactor is replaced.
In a liquid cooled nuclear reactor, the internal structure of the core barrel generally includes, in addition to the core itself, an upper internals structural package, which is installed and can be removed as a unit, and a lower internals structure which is axially aligned with the upper internals structural package. To ensure that the upper and lower internals structures are correctly oriented and aligned with one another, the inner surface of the core barrel is provided with a plurality, usually four, symmetrically disposed guide pins which are fixedly aligned with the lower internal structure and which extend radially inwardly from the inner wall of the core barrel and engage in respective slots formed in the peripheral surface of the upper core plate, which, in turn, forms the lower most portion of the upper internals structural package. In view of the relatively close tolerances required for the orientation of the upper and lower internal structures, it is customary to machine the surfaces of the peripheral slots in the upper core plate to general relatively large tolerances relative to the guide pins, and then to customize the upper core plate to the actual position and size of the guide pins by providing each of the perpheral slots of the upper core plate with an insert which has been finely machined to provide the desired small clearances, e.g. 0.013 cm (0.005 in.), based on actual clearance measurements.
In order to make these measurements when the reactor is initially being built at the factory, the upper internals structural package or unit, or at least the portion of same necessary to properly position the upper core plate, is placed in the core barrel and properly aligned with the lower internal structure including the baffle plate arrangement which surrounds the area in which the core is located. Thereafter, the various clearances between the surfaces of the peripheral slots in the upper core plate and the adjacent surfaces of the respective guide pins are actually measured by an individual, and then the upper internals structural package is removed from the core barrel. Thereafter, the actual measurements taken are used to machine the relatively small inserts which are then positioned and fastened in the respective perpheral slots of the upper core plate so as to customize the core plate to the actual positions of the lower internal structure.
Although the above technique is satisfactory when a new reactor is being built, a number of significant problems are presented when it is necessary to replace the upper internals structural package of a reactor which has been in use for some time. Initially, since the reactor vessel, including the core barrel with its guide pins and the fixed portions of lower internal structure, e.g., the baffle plate arrangement, are no longer located at the situs of the factory, it would be extremely difficult, time consuming and expensive, particularly in view of the large size and weight of even the upper core plate, which for example is in the order of 13-1400 kg (3000 lbs.) for a typical reactor, to ship the upper core plate, or a sufficient portion of the upper internals structural package, to the situs of the reactor, to take the necessary measurements, return the upper core plate to the factory for customizing, and then return the completed package to the reactor for ultimate installation. Moreover, since the reactor vessel of a previously operating reactor is somewhat radioactive and accordingly is conventionally flooded with water during the replacement or refitting period, it would be very difficult and extremely dangerous for an individual to take the necessary measurements in order to customize the new upper core plate to the existing guide pins in the core barrel.